Photovoltaic devices (also referred to as solar cells) absorb sun light and convert light energy into electricity. Photovoltaic devices and manufacturing methods therefor are continually evolving to provide higher conversion efficiency with thinner designs.
Thin film solar cells are based on one or more layers of thin films of photovoltaic materials deposited on a substrate such as glass. The film thickness of the photovoltaic materials ranges from several nanometers to tens of micrometers. Such photovoltaic materials function as light absorbers. A photovoltaic device can further comprise other thin films such as a buffer layer, a back contact layer, and a front contact layer.
Copper indium gallium diselenide (CIGS) is a commonly used absorber layer in thin film solar cells. CIGS thin film solar cells have achieved excellent conversion efficiency (>20%) in laboratory environments. Most conventional CIGS deposition is done by one of two techniques: co-evaporation or selenization. Co-evaporation involves simultaneously evaporating copper, indium, gallium and selenium. The different melting points of the four elements makes controlling the formation of a stoichiometric compound on a large substrate very difficult. Additionally, film adhesion is very poor when using co-evaporation. Selenization involves a two-step process. First, a copper, gallium, and indium precursor is sputtered on to a substrate. Second, selenization occurs by a reaction of the precursor with H2Se/H2S at 500° C. or above.